1. Field of the Invention
The present invention relates to the technical field of mobility management, and more particularly to a mobility management method and a mobility system for wireless data networks suitable for wireless data networks, such as the General Packet Radio Service (GPRS) system or Universal Mobile Telecommunications System (UMTS).
2. Description of the Related Art
In the current CPRS network, as shown in FIG. 1, the service area is divided into a plurality of routing areas (RAs) 11. Each RA is composed of a plurality of cells 12. Each cell 12 represents the coverage of a base station 13. GPRS network uses a pair of finite states machines executed in both the serving GPRS support node (SGSN) 14 and a mobile phone 15 to perform the mobility management (MM) of the mobile phone 15. The finite state machine has three states: idle state, standby state, and ready state. In the idle state, the GPRS network does not know the existence of the mobile phone 15. In the standby state, the mobile phone 15 has been attached to the GPRS network, and whenever the mobile phone 15 crosses one RA11, it is necessary to register to an associated SGSN14. In the ready state, whenever the mobile phone 15 crosses one cell 12, it is necessary to register to an associated SGSN 14. Besides, the mobile phone 15 must be in ready state for receiving and transmitting packet data units (PDUs).
By switching among the three mobility management states, the GPRS network can perform the mobility management of mobile phone 15. In the ready state, it is predicted that the mobile phone 15 has packet transfer in a short time period. Therefore, each time the mobile phone 15 crosses a cell 12, it must notify the SGSN14 of the new cell address. In this way, the SGSN14 can transfer packets to the new cell 12 directly without paging all cells 12 in the RA11. On the other hand, when one communication session between the mobile phone 15 and the SGSN14 is terminated, it is predicted that the mobile phone 15 will not have packet transfer in a short time period. In this case, the network expense is too high if the registration operation is performed whenever the handset 15 crosses one cell. Thus, the mobility management state switches into the standby state from the ready state. Accordingly, the mobile phone 15 only performs a registration whenever it crosses one RA11. That is, in the ready state, the packet can be sent to the mobile phone 15 directly without extra paging cost. However, the cost of location update is very high since the mobile phone 15 has to perform cell updates. In the standby state, the cost of paging is high since it is necessary to page all cells 12 of the RA11 in which the mobile phone 15 resides. However, the cost of location update is low since the mobile phone 15 only performs RA updates.
As to the above switch from the ready state to the standby state, a ready timer approach is employed in the 23.060 specification of 3 GPP. This approach defines a threshold interval, denoted as T. When a packet is transferred, the ready timer starts to count time reversely through T time units. If the ready timer has completed the counting and the mobile phone 15 has no data transfer, the mobility management state switches from the ready state to the standby state.
With use of the above ready timer approach to switch the mobility management state from the ready state to standby state, if the mobility pattern of the mobile phone 15 and the traffic pattern change frequently, the timing of switch from the ready state to the standby state can not be determined precisely since the ready timer determines the switching time by the fixed threshold interval T. Besides, the ready timer approach will encounter a problem of lose synchronization. For example, when the SGSN 14 has switched to the standby state, the mobile phone 15 is still in the ready state.
In the U.S. Pat. No. 6,243,579, “Controlling Operating States of a Mobile Station in a Packet Radio System”, three extra messages are added in the GPRS network to request performing the switch of mobility management states. Such an improvement is aimed at the ready timer. Therefore, the defect of being unable to precisely determine the switching time from the ready state to standby state due to using the fixed threshold interval T still exists. Moreover, the synchronization problem between the mobile phone 15 and SGSN14 can not be avoided.